1. Field of the Invention
This invention relates generally to solar water-heating-systems, and more specifically to an improved solar water-heating-system having a collector core which includes heating-pipes in each of which countercurrent flows of hot and cold fluid pass in opposite directions.
2. Description of the Prior Art
A solar water-heating-system to which the present invention particularly relates comprises a solar water-heating-panel connected to a hot-water storage-tank. Such a solar water-heating-panel frequently includes vertically oriented, parallel heating-pipes or other channels embedded in a selective absorber, such as a black metal sheet, that is inclined towards the sun. The heating-pipes generally open into horizontal manifolds or header pipes both at the top and at the bottom of the solar water-heating-panel. Sunshine heats up the selective absorber and the pipes or other channels embedded in the selective absorber. This heat is transferred by conduction to a fluid, usually water, in the heating-pipes. Upon heating, the water expands slightly so its density becomes less than that of cooler water in the other parts of the solar water-heating-system. The hotter water then rises toward the top of the inclined panel and enters the horizontal upper manifold. The heated water then rises further from the upper manifold through a bend of one or two 90 degree elbow joints before entering the hot-water storage-tank. The new incoming hotter water upon entering the hot-water storage-tank pushes its way to the top, displacing cooler water already present in the hot-water storage-tank. The displaced cooler water first sinks to the bottom of the hot-water storage-tank, after which it continues to sink down a cooler-water return-pipe located at one side of the hot-water storage-tank and down the side of the solar water-heating-panel, makes another 90 degree turn in an elbow joint, to then enter the horizontal manifold at the bottom of the solar water-heating-panel. The cooler water, after entering the manifold at the bottom of the solar water-heating-panel, is heated again by the sun's rays and the process begins anew. Thus, the temperature of water in the hot-water storage-tank increases throughout the day.
Existing thermosyphon based solar water-heating-systems of this kind normally consist of a solar water-heating-panel with a separate insulated hot-water storage-tank placed immediately above the water-heating-panel. An example of a prior art of solar water-heating-system of this type is disclosed in U.S. Pat. No. 4,084,578 that issued Apr. 18, 1978, on an application filed by Toshihiro Ishibashi ("the Ishibashi patent"). A drawing depicting the solar water-heating-system disclosed in the Ishibashi patent is included herein as FIG. 0. The Ishibashi patent discloses increasing collection efficiency by improving the selective surface of the absorber through special paints and coatings, special non-reflecting glass, utilizing different corrugation profile angles for the collector sheet, and using the hot-water storage-tank as a reflector in winter.
Placement of the hot-water storage-tank in close proximity to and immediately above the solar water-heating-panel is known to be advantageous as disclosed in U.S. Pat. No. 4,766,885 that issued Aug. 30, 1988, on an application filed by Toshiaki Muramatsu (the Muramatsu '885 patent"). However, in solar water-heating-systems such as that disclosed in the Muramatsu '885 patent, the hot water must flow horizontally across the breadth of the solar water-heating-panel, or even worse, across two panels if it is a two panel system, before entering the hot-water storage-tank. Moreover, before entering the hot-water storage-tank, the hot water must also flow through one or two elbow joints with all their attendant increase in resistance to flow due to form drag, i.e. eddying and turbulence, and friction drag etc. which impedes thermosyphon flow.
It is well known that slowing down natural thermosyphon flow reduces the efficiency of the heat collection because the water in the horizontal manifold becomes trapped, unable to move in its natural upwards direction. Consequently, water in the horizontal manifold gets hotter and hotter as it continues to absorbs solar radiation. This relatively stagnant flow of hot water in the upper manifold, in the upper part of the heating-pipes, and in the elbow joints becomes disadvantageously hot, and radiates away heat through glass covering the solar water-heating-panel. Furthermore, some heat energy of the rising hot water in the vertical heating-pipes is also lost or expended in pushing the flow along the horizontal manifolds. All of these losses reduce the overall heat transfer coefficient of efficiency.
U.S. Pat. No. 4,353,352, that issued Oct. 12, 1982, to Michael F. Zinn ("the Zinn patent"), discloses an improved thermosyphon flow having a near direct connection from the heating-pipes to the hot-water storage-tank. However, solar water-heating-system disclosed in the Zinn patent forces the hot water to travel in a roundabout way from the top of the panel, curving behind the tank, before entering the tank itself. Furthermore, the Zinn patent also shows that upon entering the hot-water storage-tank the hot water must also flow downwards, against thermosyphon flow, because the outlets of the hot water inlet-pipes are located at the very top of the tank, pointing downwards. Placing the inlet-pipes at the top of the hot-water storage-tank causes hot water to build-up in this area and to become congested once some hot water has accumulated at the top of the tank. This congestion occurs because any new incoming hot water must drive the existing layer of hot water downwards within the hot-water storage-tank. Since this layer of hot water naturally resists flowing downward, the "plug" of hot water around the top of the tank effectively slows down the thermosyphon flow even more. Again, thermal collection inefficiency rises as the amount of hot water increases.
Known prior art solar water-heating-systems, including those disclosed in the Ishibashi and Zinn patents have the cooler-water return-pipe located at the side of the water-heating-panel. This location for the cooler-water return-pipe impedes the thermosyphon flow since horizontal runs, which impede the natural upwards or downwards movement of thermosyphon flow, are then necessary across the width of the solar water-heating-panel(s). In prior art solar water-heating-systems, not only does the cooler water, which wants to sink, have to travel horizontally across the length of the hot-water storage-tank before finding the outlet leading down to the panel, but upon reaching the bottom of the panel, the cooler water must travel horizontally back across the width of the panel before reaching the furthest heating-pipe. Thus, in prior art thermosyphon solar water-heating-systems only a few of the heating-pipes near the lower manifold'ms inlet and the upper manifold's outlet operate at peak heat collecting efficiency. Consequently, a significant portion of a solar water-heating-panel located at a distance from the manifolds' inlet and outlet experience stagnant or retarded flow.
U.S. Pat. No. 4,724,826, that issued Feb. 16, 1988, on another application filed by Toshiaki Muramatsu ("the Muramatsu '826 patent"), as well as the Muramatsu '885 patent, disclose a two phase system in which a working-fluid, gasified by solar radiation in an evaporator portion of a heat pipe, conducts heat to a condenser portion of the heat pipe where the working fluid returns to the liquid phase. In the solar water heaters disclosed in the Muramatsu '885 and '826 patents, gaseous heat-conducting fluid rises upwards while condensed heating-fluid descends downwards countercurrently within a multiplicity of hermetically sealed parallel channels included in a plate-like, solar heating-panel. In the systems disclosed in the Muramatsu '885 and '826 patents, the heating-fluid, apparently Freon or a like working fluid sealed within the plate-like heat absorber, must be isolated from the water to be heated in the hot-water storage-tank. Accordingly, in the systems disclosed in the Muramatsu '885 and '826 patents the water in the storage-tank is indirectly heated through conduction while passing through a heat exchanger which also contains the working-fluid, or an intermediate working-liquid heated by the gaseous first working-fluid.
Existing prior art solar hot water panels are also susceptible to mechanical damage if water in the heating-pipes freezes and cracks the heating-pipes. Some prior art systems have addressed this problem through an indirect system in which solar radiation heats an antifreeze solution in heating-pipes, or is heated at the condenser portion of heat pipes. The hot antifreeze solution then circulates through a heat exchanger located in the hot-water storage-tank to heat the water. This type of solar water-heating-system is inefficient since the water is only indirectly heated by the antifreeze solution. Consequently, this indirect solar water-heating-panel system heats less water than a direct solar water-heating-system in which the water being heated circulates through heating-pipes. Moreover, indirect solar water-heating panel systems are more expensive and complicated that a direct solar water-heating-system, and require maintenance including regular topping up of any intermediate-working liquid antifreeze solution if such is used.